I'm looking for a very simple measurement circuit to measure vibrations with a piezo elment and an arduino device. I want to read positive and negative values.
I have this sensor: http://www.seeedstudio.com/wiki/Grove_-_Piezo_Vibration_Sensor
I know that this sensor module only gives me digital output. But I want to measure the vibration amplitudes and the frequency. So I would use just the piezo element.
I found a lot examples, from very simple to high accurate. But most of them cutting of the negative values. E.G. http://www.learningaboutelectronics.com/Articles/Piezo-knock-sensor-circuit.php
And in the example above, there is no protection for the arduino. I also found statements, that the arduino needs no protection reading a piezo sensor at analog pin, because of the low current. And the analog pins are protected against higher voltages.
I want just a simple circuit to read negative and positive values from that piezo element. My arduino device use 3.3V input voltage. The piezo sensor can have in extrem up to 70-90V.
What is the minimum configuration to set up such a measurement circuit? I don't need highest accuracy. It should be as easy as possible. It is just for test purpuses. I think I have to use an voltage divider with two equal high resistences to set a DC offset for measuring the negativ values. But what else do I need for a minimal working example?